This invention relates to a printer operating on the principle of thermal ink transfer, such that ink is thermally transferred from an ink transfer sheet to an ink recording sheet for reproduction of a desired image either in color or in black and white. More particularly, the invention concerns an improved mechanism in such a printer for pressing a thermal printing head against a platen via a superposition of the transfer sheet and the recording sheet. The printer with the improved head pressing mechanism is intended for use with an insertable cassette in which are housed both a stack of recording sheets and an elongate strip of transfer sheet.
The thermal ink transfer printer has won extensive commercial acceptance for the production of hard copies (i.e. those readable or appreciable without use of special device) of computer graphics or the like. Among the primary reasons for this popularity are simplicity in construction, fast printing speed, and ease of handling.
Basically, in the printer of this type, the recording sheet is printed upon while being pressed against a platen by the thermal printing head via the transfer sheet. The platen is driven as by a direct current motor or a pulse motor. The printing head has a plurality of electric heater elements which are aligned lengthwise of the transport roller. The electric current fed to the heater elements is controlled for printing dots on the recording sheet line by line with the incremental rotation of the platen.
For color printing, a color transfer sheet is employed which bears on its different sections the inks of the three primary colors, yellow, magenta and cyan. A black ink may also be used to add detail and contrast to the printed reproduction, as is well known in the printing art. The ink of a first preselected color is first transferred from the color transfer sheet to the recording sheet by automatically holding the recording sheet in register with the required ink section of the transfer sheet. Then the transfer sheet is fed a required distance for transferring the ink of a second preselected color to the recording sheet. The same procedure is repeated on the same recording sheet for each additional color until a full color printing is completed.
Of course, the color transfer sheet needs replacement as the inks are used up, and a new recording sheet is required for each printing operation. A known solution to the problem of how to overcome troubles with frequent transfer sheet replacement and recording sheet replenishment is a cassette in which there are housed both a color transfer sheet in the form of an elongate strip and a stack of recording sheets. The cassette can be readily inserted in the associated printer. The color transfer strip extends between a payoff and a takeup mandrel within the cassette. The insertion of this cassette in the printer serves the dual purpose of replacing the used transfer sheet and introducing a fresh supply of recording sheets.
The printer for use with the insertable cassette is constructed to take out the transfer strip takeup mandrel from within the cassette and to hold it in a preassigned position within the printer. Being anchored at one end to the takeup mandrel, the transfer strip is thus pulled out of the cassette and held against the platen. Also, the recording sheets are withdrawn one by one from within the cassette, introduced into the printer, and clamped against the platen preparatory to thermal ink transfer from the transfer sheet by the printing head.
Japanese Utility Model Application No. 63-31345 is hereby cited as a prior art printer designed for use with an insertable cassette, to which the present invention bears particular pertinence. This prior art printer has proved to be in need of improvement in regard to its mechanism for pressing the thermal printing head against the platen via the superposed transfer strip and recording sheet. The known head pressing mechanism has comprised a dedicated rotary solenoid together with an associated drive linkage. A drawback of this known mechanism is that it has unnecessarily increased the number of constituent parts of the printer and thus added to its manufacturing cost.
A particular objection is to the rotary solenoid, which is unduly expensive in consideration of the function for which it is intended. Moreover, the rotary solenoid must of necessity be inconveniently large in size for pressing the printing head with a sufficient force to assure high quality printings. Such a bulky rotary solenoid has demanded a correspondingly large installation space, with a consequent increase in the overall size of the printer itself. Additional difficulties with the rotary solenoid have been its great power requirement and much noise production.